In continuous casting processes molten metal is poured at a steady rate into a cooled mold. Typically, a shell forms by solidification of the metal along the mold wall. The casting is withdrawn from the bottom of the mold. The solidified shell acts to contain the molten metal inside the shell. The casting emerging from the bottom of the mold is sprayed to cool and solidify the metal further.
A chill can be used to increase the rate of cooling of the casting. A chill is typically a metal or graphite insert placed in a mold to rapidly cool and solidify the casting, producing a hard surface.
One type of continuous casting mold including a chill has an upper superstructure positioned above the chill. The inner wall (facing the molten metal) of the superstructure projects closer to the molten metal than the inner wall of the chill to form an overhang over the chill where the chill meets the superstructure. In use, the inner wall of the chill is cooled and lubricated, and a gas under pressure is introduced to the mold cavity at the point where the superstructure meets the chill.
The method and apparatus described above is disclosed in DE-OS 27 34 388, to Showa, published Feb. 2, 1978. The method described in the Showa reference leads to fairly smooth billet surfaces only under favorable conditions. When starting to cast, for example, monitoring and regulating the pressure of the gas and lubricating oil flow is necessary. In addition, continuous temperature measurements must be made to adjust the casting parameters. In practice, the need for continuous monitoring and adjustment often leads to difficulties in producing satisfactory castings. These problems are compounded by fluctuations of the metal level and changes of the metallostatic pressure in the running casting process.
Further, it has been found that with the method according to DE-OS 27 34 388 lead-containing alloys containing up to 2.5% lead cannot be cast satisfactorily. These alloys are of special importance in the manufacture of continuous casting products to be machined by chipping.
Experiments with gaseous lubricants like acetylene, butadiene, propane and trichlorethylene have shown that, under the pressure and temperature conditions prevailing in the chill, it is not possible to obtain suitable lubrication making use of the decomposition of the gaseous lubricants. When conventional lubricants without gas are used, by introduction parallel to the chill axis, lubricant combustion will occur in the starting phase, (meniscus cavity) and subsequent underdosing of lubricant will occur along the running surface of the chill. It is therefore, not possible to establish optimal lubrication by lubricant dosing only.